Force sensitive resistor

ABSTRACT

A force sensitive resistor (10) is provided comprising: a flexible bottom layer (8) of a first material, comprising a first conductive element attached to the bottom layer (8); a spacer ring (18) attached to the bottom layer (8), the spacer ring (18) surrounding the first conductive element and extending away from the bottom layer (8); and a flexible top sensor layer (19) of a second material attached across the spacer ring (18) comprising a second conductive element facing the first conductive element. The flexible top sensor layer (19) is moveable relative to the flexible bottom layer (8) to vary the resistance of the force sensitive resistor (10). The Young&#39;s Modulus of the first material is less than the Young&#39;s Modulus of the second material. The flexible bottom layer (8) extends past the spacer ring (18).

The present invention relates to a force sensitive resistor for use with stretchable materials.

Traditionally, force sensitive resistors are provided with a lower conductive member formed on a first substrate, and an upper conductive member formed on a second substrate. A spacer element is mounted between the first and second substrates. As the conductive members are moved toward and away from each other the resistance of the force sensitive resistor is altered. In this manner the force applied to the resistor can be determined by the change in the resistance. The substrates are required to be relatively stiff and resist stretching as otherwise the data from the force sensitive resistor will be corrupted.

This is particularly relevant as force sensitive resistors are often used in environments which receive significant forces and deformations. In particular, force sensitive resistors are being used in so-called “smart” clothing. This includes soles or inner soles for shoes which are used to generate data regarding the user's gait, pressure distribution and/or pronation.

With the invention of conductive inks, the ability to print electrical circuits directly on to materials has been developed. It is possible to directly print one of the conductive members onto the surface where the force is to be detected. However, as the material stretches and flexes the data will become unusable. There is therefore a need to develop a system that allows direct application of a force sensitive resistor to a flexible and stretchable surface.

A force sensitive resistor is provided according to the present invention according to claim 1. The force sensitive resistor comprising: a flexible bottom layer of a first material, comprising a first conductive element attached to the bottom layer; a spacer ring attached to the bottom layer, the spacer ring surrounding the first conductive element and extending away from the bottom layer; and a flexible top sensor layer of a second material attached across the spacer ring comprising a second conductive element facing the first conductive element, the flexible top sensor layer being moveable relative to the flexible bottom layer to vary the resistance of the force sensitive resistor, characterised in that the Young's Modulus of the first material is less than the Young's Modulus of the second material. The flexible bottom layer extends past the spacer ring.

This locally limits the amount of flexion of the lower layer in the region surrounding the first conductive element. In this way, accurate data can be obtained from a force sensitive resistor provided on a flexible and stretchable layer. The ring locally resits deformation in the larger flexible bottom layer to help reduce distortion of the signal caused by flexion of the layer.

Preferably, the first conductive element is formed of a conductive ink printed onto the flexible bottom layer. Conductive ink can be readily applied to a flexible layer easily.

The bottom layer of the force sensitive resistor may be a garment. A garment will naturally flex with the wearer. The force sensitive resistor of the present invention reduces distortion of the signal caused by this deformation.

The garment may further comprise one or more additional force sensitive resistors formed in an array. An array of force sensitive resistors can allow for complex force profiles to be generated.

The bottom layer of each force sensitive resistor may be a part of one continuous layer of the garment. This is a good way to distribute force sensitive resistors across the garment, by applying them directly to one of the layers thereof.

The bottom layer of each force sensitive resistor may be arranged to contact a wearer in use. The ring then acts to limit deformation in the layer which contacts the wearer which helps ensure accurate signals.

The garment may be a sole or inner sole. Force readings from a sole or inner sole may be of great interest. In particular runners may be interested in these readings.

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a top schematic view of a shoe insole comprising a number of force sensitive resistors according to the invention;

FIG. 2 is an exploded schematic, part plan, part perspective, view of a force sensitive resistor according to the invention;

FIG. 3 is a side sectional view of the force sensitive resistor of FIG. 2;

FIG. 4 is a sectional view along the line A-A of FIG. 1;

FIG. 5 is a view of a cushioning material for use in the sole of FIG. 1;

FIG. 6 is a sectional view along the line B-B of FIG. 1;

FIG. 7 is an exploded schematic view of a force sensitive resistor according to the prior art;

FIG. 8 is a top schematic view of a shoe insole comprising a number of force sensitive resistors according to a further embodiment of the invention; and

FIG. 9 is a top schematic view of an alternative sole insole comprising a number of force sensitive resistors according to the invention.

The sole (or insole/inner sole) 100 shown in FIG. 1 is formed of upper and lower flexible and stretchable layers 8, 9 (shown in FIG. 4) which are sealed in a water and air tight manner around their outer edge. While the term “sole” is used throughout the description, the described embodiment likewise may apply to an inner sole or insole. In particular, the layers 8, 9 may be sealed by means of heat or sonic welding. The sole 100 may be a separate, removable component, or may be integrated with a shoe. The sole 100 is shaped for a human foot and the embodiment of FIG. 1 defines two reference directions; a longitudinal direction X extending from a heel region 22 towards the back of the sole 100 to a toe region 28 towards the front of the sole 100, and a lateral direction Y extending from the inner side to the outer side of the sole 100. While the sole shown in FIG. 1 is for a right foot, it is appreciated that a mirror version of this sole would be suitable for a left foot. Alternatively, the sole 100 may be usable for either foot by turning the sole 100 over.

Moving from the heel region 22 in the longitudinal direction, the sole 100 has a midfoot region 24, a forefoot region 26 and a toe region 28. In use, the heel region 22 supports the user's heel, the midfoot region 24 supports the user's arch, the forefoot region 26 supports the user's forefoot and the toe region 28 supports the user's toes.

The sole 100 is provided with a number of force sensitive resistors 10 arranged across the sole 100. An exemplary force sensitive resistor 10 is shown in FIG. 2. Conductive ink is printed on the bottom layer 8 in a pattern for a force sensitive resistor 10. This conductive ink forms electrodes 12, 14. These electrodes 12, 14 are substantially semi-circular with prongs 13 extending in alternating rows between the electrodes 12, 14. This forms a first conductive element. The prongs 13 have been omitted from the rest of the Figures for clarity, but they are present in each force sensitive resistor 10.

The bottom layer 8 is generally a small section of a larger sheet forming a larger structure. In particular, the bottom layer 8 may be a fabric, plastic or other flexible material which forms a part of a garment. While the present invention is generally described with respect to a sole 100, it is appreciated that it may be used with any other garment.

A garment is generally intended to mean anything which may be worn by a person. In particular, the garment may be any of a top, vest, trouser, jacket, helmet, inner sole, shoe, under-garment, or any other garment.

By printing the conductive ink onto this bottom layer 8 a force sensitive resistor 10 may be provided on the garment. The flexible bottom layer 8 may then contact a wearer and/or be subjected to an external impact force, and the force sensitive resistor 10 can determine the force applied by the wearer.

A spacer ring 18 is provided surrounding the first conductive element. A top sensor layer 19 is provided across the spacer ring 18. The top sensor layer 19 is flexible and comprises a second conductive element. Typically, the top sensor layer is formed from PET. The first and second conductive elements may be moved relative to one another in order to vary the resistance of the force sensitive resistor 10 as the user runs.

As the conductive ink is printed directly onto the stretchable lower layer 8, the output of the force sensitive resistor 10 may be altered when the lower layer 8 flexes and stretches and hence the results from the resistor 10 cannot be practically used. In order to address this, the upper sensor layer 19 is stiffer than the lower layer 8. This provides the localised region of the lower layer 8 with enhanced strength, on which the first conductive element is printed. In particular, this is achieved by the upper sensor layer 19 having a higher Young's modulus than the lower layer 8. This locally limits the ease of stretching of the lower layer 8 in the region of the first conductive element within the spacer 18. As such, the first conductive element may be printed directly on to the lower flexible layer 8 whilst still obtaining useful data.

As shown in FIG. 1, the lower layer 8 is provided with printed conductive tracks connected to the electrodes 12, 14 of the force sensitive resistor 10. As shown, there may be a single ground track 15 to which each force sensitive resistor 10 is connected. This reduces the space required for the tracks. Each force sensitive resistor 10 is also provided with its own data track 16. The sole is provided with a tab 7 which provides a pathway for the conductive tracks 15, 16 to an external CPU and control system (not shown). The tab 7 is integral with the bottom layer 8 and is likewise flexible and stretchable. While the present figures are shown with an external CPU and control system, this is not necessarily the case, and the CPU and control system may be integral with the sole 100 or provided in another layer designed to be contained within the shoe with the sole 100.

The upper and lower flexible layers 8, 9 of the sole 100 are also sealed in a water and air tight manner across a portion of the central region of the sole 100. This forms a central sealed region 3 on the sole which may extend across the heel region 22, the midfoot region 24 and the forefoot region 26.

As shown in FIG. 4, which is a schematic view along A-A as shown on FIG. 1. Between the outer seal and the central sealed region 3, a compressible material 5 is provided. The compressible material 5 is designed to not seal to either of the upper and lower flexible layers 8, 9. In particular the layers 8, 9 should not sonic weld to the compressible material 5. The compressible material 5 is free between the upper and lower flexible layers 8, 9. In particular, the upper and lower flexible layers 8, 9 are not bonded to the compressible material 5. This leaves the compressible material 5 free to move in the gap between the layers 8, 9. As the compressible material 5 is substantially continuous, it is held in position by the central sealed region 3 as shown in FIG. 1.

The compressible material may be any suitable material. In particular embodiments it is a foam material, with either an open cell or closed cell arrangement. FIG. 5 shows an exemplary open cell foam which may be used with the present invention. High resilience to deformation is necessary to ensure that the sole 100 is not permanently deformed in use. The material 5 may form the main support material of the sole 100, or an additional cushioning material may also be provided for enhanced comfort. The material 5 is selected to have a higher melting point than the material of the first and second layers 8, 9 such that the layers 8, 9 will seal to one another around the material 5 without sealing to the material 5.

The material 5 may be provided across multiple force sensitive resistors 10. That is, the material 5 may be shared by a plurality of force sensitive resistors 10.

As shown in FIG. 6 which is a sectional view along B-B, in preferred embodiments the material 5 is provided with cutaway sections substantially aligned with the force sensitive resistors 10. In particular, these cut-away sections may be formed such that the material 5 substantially surrounds the force sensitive resistor 10. The material 5 may then not have any overlap with the force sensitive resistor 10.

Air is able to flow through the material 5. This allows the entirety of the sole 100 to be sealed around its outer edge. Typically, prior art force sensitive resistors 50 as shown in FIG. 7 which are formed of first conductive element 51, spacer 53 and second conductive element 55. The spacer 53 is provided with a vent pathway 57 extending from the spacer chamber 58 extending from the spacer chamber 58. The spacer chamber 58 is provided between the conductive elements.

As the conductive elements are moved towards and away from one another, the volume of the spacer chamber 58 is varied. Accordingly, air which is held in the spacer chamber 58 must be expelled via the vent pathway 57 otherwise the prior art force sensitive resistor 50 may rupture. As such, in prior art soles without the air permeable compressive material 5 of the present invention, a vent pathway must be provided from the force sensitive resistor 50 to the atmosphere outside of the sole 100. While efforts are made to minimise these vent pathways, they represent pathways via which moisture may ingress and damage the sole 100.

By providing the air permeable material 5, these vent pathways are no longer necessary. The air displaced by movement of the force sensitive resistors 10 can be distributed across the sole 100 without risking rupture. This allows the entire sole 100 to be sealed to the atmosphere, which ensures better moisture resistance than known systems.

FIG. 9 shows an alternative embodiment of a sole 1. The wiring of the earlier embodiments is not shown in from this Figure for ease of understanding. The wiring of this embodiment is substantially the same as described with the other embodiments. In this embodiment, the compressible material 5 does extend generally across the sole but instead is concentrated in regions around the force sensitive resistors 10 in circular regions 92 surrounding each force sensitive resistor. As the compressible material 5 has a higher melting point than the material of the sole, a seal 98 will be formed around the shape of the compressible material 5.

In the embodiment of FIG. 9, connecting portions 94 of compressible material 5 join the circular sections 92 to form a continuous region of compressible material 5. In the toe and forefoot regions 28, 26 the circular sections 92 may have some degree of overlap 96.

In the embodiment shown in FIG. 9, the compressible material 5 is continuous across all of the force sensitive resistors. However, this is not necessarily the case. In other embodiments particular regions may each have their own discrete regions of compressible material 5. For example, there may be one discrete region in the toe region 28, one discrete region in the forefoot region 26 and two discrete regions down either side of the sole in the heel and midfoot regions 22, 24. Alternatively, each force sensitive resistor 10 may have its own discrete sealed section of compressible material 5. There may also be a combination of individual discrete sealed sections and discrete regions of compressible material 5. For example, the toe and forefoot regions 28, 26 may each have their own discrete region of compressible material 5, while the force sensitive resistors 10 in the heel and midfoot regions 22, 24 may be in individual discrete sealed sections of compressible material.

The force sensitive resistors 10 are distributed throughout the sole 100. This ensures that a detailed understanding of the user's weight distribution can be calculated, along with information such as the degree of pronation of the user while walking. In particular, as shown in FIG. 1, each region 22, 24, 26, 28 comprises an individual sensing area comprising a plurality of force sensitive resistors 10. These force sensitive resistors 10 are generally similar in shape to one another, in that they are each substantially circular. In each of the toe and forefoot regions 28, 26 a row of force sensitive resistors 10 extending laterally Y across the sole 100 are provided. In particular, each row comprises five force sensitive resistors arranged to acquire data regarding each toe and corresponding toe knuckle.

While this arrangement in the toe and forefoot regions 28, 26 is ideal it may not be possible in all embodiments. In particular, for soles 100 designed for smaller feet it may not be possible to fit five separate force sensitive resistors 10 across the lateral direction Y. Accordingly, an arrangement such as that shown in FIG. 8 may be provided. As shown, the innermost three force sensitive resistors 10 are provided as in FIG. 1. However, instead of two separate outer force sensitive resistors 10, a single force sensitive resistor 40 is provided. This force sensitive resistor 40 is elongated in the lateral direction Y to cover the outer part of the toe and forefoot regions 28, 26. This elongated force sensitive resistor 40 is substantially rectangular with rounded ends in FIG. 8. However, any suitable shape may be used. The force sensitive resistor 40 is provided with electrodes 42, 44 and functions as the circular force sensitive resistors 10 described above. It has been identified that the data from the individual outer toes is not of as much interest as the inner regions. Accordingly, the data from the outermost toes can be combined while still being useful for analytics.

Across the heel and midfoot regions 22, 24 two longitudinal rows of force sensitive resistors 10 are provided. One row is on the inner side of the sole and the other row is on the outer side, with the sealed central region 3 provided between these rows. At least one force sensitive resistor 10 is provided in each row in each of the heel and midfoot regions 22, 24. Preferably, if the sole 100 is big enough, each row has two force sensitive resistors 10 in each of the heel and midfoot regions 22, 24.

By providing force sensitive resistors 10 in this arrangement a high quality of data can be obtained that provides a lot of information regarding the weight distribution and pronation of the user. 

What is claimed is:
 1. A force sensitive resistor comprising: a flexible bottom layer of a first material, comprising a first conductive element attached to the bottom layer; a spacer ring attached to the bottom layer, the spacer ring surrounding the first conductive element and extending away from the bottom layer; and a flexible top sensor layer of a second material attached across the spacer ring comprising a second conductive element facing the first conductive element, the flexible top sensor layer being moveable relative to the flexible bottom layer to vary the resistance of the force sensitive resistor, wherein: the Young's Modulus of the first material is less than the Young's Modulus of the second material; and the flexible bottom layer extends past the spacer ring.
 2. A force sensitive resistor according to claim 1, wherein the first conductive element is formed of a conductive ink printed onto the flexible bottom layer.
 3. A garment comprising a force sensitive resistor according to claim 1, wherein the bottom layer of the force sensitive resistor is a layer of the garment.
 4. A garment comprising two or more force sensitive resistors according to claim 1, wherein the force sensitive resistors are formed in an array, and wherein the bottom layer of each force sensitive resistors is a layer of the garment..
 5. The garment of claim 4, wherein the bottom layer of each force sensitive resistor is a part of one continuous layer of the garment.
 6. The garment of claim 3, wherein the bottom layer of the force sensitive resistor is arranged to contact a wearer in use.
 7. The garment of claim 3, wherein the garment is a sole or inner sole. 